The Cygnus Loop – A bicolor l-enhance mosaic

This 2-panel mosaic portaits the entire nebular complex known as the Cygnus Loop, including the famous Eastern and Western Veils.

The Subject

This beaufitul complex includes some of the most beautiful nebulae of the Northern sky. The subtle and elaborate details are exalted by narrowband shots, due to the rich  and localized Hydrogen-alpha and Oxygen-III emissions.

This is a very large object, and in order to frame it in its entirety, even with my widefield rig, a 2-panel mosaic was necessary. These are the two panels and the mosaic: i’ve had no problems creating the mosaic with Pixinsight.

The Shooting

For this shot i tested a new location, with a less polluted sky than my home observatory. My mother’s and his partner place is located about 15 km from the city of Arezzo, in a little-inhabited valley. While the summer Milky Way from my home is barely visible even at the zenith (bortle 7?), here, even in these very warm and humid August nights, it’s visible in its entirety, and at the zenith shows interesting details. I failed to spot M33 with the naked eye, but i think it should be visible on the best nights. Taking all these into account, the sky here should be between Bortle 5 and 6.

Here are the specifications of my widefield rig, I’m using an old but functional Canon EOS 1100D, with a full-spectrum astro modification and an Optolong L-Enhance double narrowband filter.

Also this time i used 10 minutes subs at ISO1600. In order to collect enough data i needed 4 different sessions. I alternatively shot both the two panels on each session, switching panel at the meridian flip . The collected data it’s not bad, but unfortunately i’ve had focusing troubles and probably a tilt somewhere in the image train, and the star quality is not very good.

0. Pre-Processing

As usual for preprocessing I used PixInsight‘s WeightedBatchPreprocessing script, leaving most of the options as default, with an entire set of calibration frames including bias, flat and dark. Nothing really fancy here…

1. Extraction and processing of the single channels

I extracted and processed the hα and Oiii channels of each panel separately as show in my previous post. I’m quite satisfied of the result, despite the workflow is somewhat burdensome and it’s difficult to effectively reduce halos around bright stars.

The following picture shows a 1:2 crop of the  channel of the first panel. The other channel and panel were processed in a similar way. The workflow includes Deconvolution, Star Reduction and a two-step Noise Reduction.

2. Creating the mosaic

I created the mosaic using this tutorial from LightVortexAstronomy:
https://www.lightvortexastronomy.com/tutorial-preparing-a-mosaic.html
It’s an easy procedure and, given the wide field and the large amount of stars in the picture, StarAlignment had no problems solving and aligning the mosaic.

3. Stretch

I stretched the image using the MaskedStretch process: this method allows to fully enhance the faint details of the nebula. without bloating the stars.

4. LocalHistogramEqualization

The LocalHistogramEqualization process makes and excellent work in further enhancing the fine details and filaments of the Veils. The effect is clearly visible in the following crop.

5. Curve and Black Point correction

I used the CurvesCorrection to adjust general contrast and tonality of the image, and then HistogramTransformation the set the black point. A few corrections were needed, since the image is already quite balanced.

6. Final noise reduction and correction

The image needed a final noise reduction, using the ACDNR process, and i tried to reduce the stars a little more using a contour mask. I’m quite satisfied of the result so i exported the image on Photoshop for the publication.

6. Photoshop correction

I used Photoshop to bring the general contrast and black point at the desired result, but little correction was needed.

This is the final image, at a resolution of 2000×2000 pixels.

Conclusions

Given the time and efforts i’ve made in the making of this shot i was aiming for a slightly better result. However, i think i’ve learned a few tips and tricks that will improve my future projects.

In particular, i have to carefully check my image train for tilt and improve the automatic focusing process, so i can always have good stars, even during long automated sequences.

In the meantime, as always, thank you for reading and clear skies 😉

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